Systems which provide terrain contour mapping (TERCOM) have been used in the past to provide position location information for moving platforms such as aircraft and missiles. For example, U.S. Pat. No. 5,170,171, incorporated herein by reference in its entirety, describes a system for three-dimensional interferometric synthetic aperture radar terrain mapping employing altitude measurement. In such systems, synthetic aperture radar data is used in combination with separately generated altimeter data to produce a terrain map corrected for platform roll angle. In general, such systems use two synthetic radar antennas and a ranging altimeter located on the aircraft.
Another system which is able to provide a terrain map is disclosed in U.S. Pat. No. 5,260,708, also incorporated herein by reference in its entirety. In this system, a three-dimensional interferometric synthetic aperture radar terrain mapping is described in which unambiguous phase unwrapping employing subset bandwidth processing is used. As with the former system, this system employs two radar receivers which process echo signals conventionally to yield slant range and doppler frequency data for plural resolution cells. The measured phase difference for each resolution cell provides an ambiguous measure of slant range difference to the two antennas needed to determine terrain evaluation and correct ground range. The received echo data is reprocessed using less than the entire bandwidth of the radar transmission to achieve additional center wave lengths. This produces a differing ambiguity interval and permits unambiguous determination of the slant range difference. The average altitude is separately determined by a ranging altimeter.
Other topographical mapping radar systems, such as disclosed in U.S. Pat. No. 4,359,732, incorporated herein by reference in its entirety, use two vertically spaced fan beam antennas to provide terrain contour mapping. This radar transmission is normal to the flight path and to the side of the aircraft.
Although such systems have been effective in mapping terrain, their accuracy is somewhat limited by the accuracy of the separately generated altimeter information. With the advent of real time digital signal processing, there is a need for an improved system which can accurately determine the height of a platform above the surface so that this information can be used to more accurately provide contour mapping.